Apache James - Implement Thread support for JMAP

JMAP is an email application protocol to modernise IMAP, on top of HTTP using a JSON format. JMAP is designed to make efficient use of limited network resources and to be horizontally scalable to a very large number of users. Apache James is one of the first implementations of this new standard.

Mail user agents generally allow displaying emails grouped by conversations (replies, forward, etc...). As a part of JMAP RFC-8621 implementation (https://tools.ietf.org/html/rfc8621#page-20), there are dedicated concepts: threads. JMAP Threads is already implemented In Apache James in a rather naive way: each email is a thread of its own. This naive implementation is specification compliant but defeats the overall purposes of threads: emails which related to a topic should belong to a thread.

James’s data models, storage APIs, and some JMAP methods at HTTP level need to be changed to make sure the purpose of the thread is reached.